1. Field of the Invention
The present invention relates to an activated cathode and a method for manufacturing the activated cathode, and more particularly to an activated cathode which is capable of electrolyzing solution of alkali metal salt such as alkali-metal halide, alkali-metal hydroxide or the like with a low hydrogen overvoltage for a long term.
2. Description of Related Art
Materials which contain soft iron as a main component have been hitherto used for cathodes in an electrolysis process for electrolyzing alkali-metal halide solution or alkali-metal hydroxide solution by using a diaphragm method, an ion-exchange membrane method or the like. In this electrolysis process, hydrogen is generated on the cathodes. Soft iron has a disadvantage that its hydrogen overvoltage is high. Therefore, there has been proposed various types of activated cathodes each having a hydrogen overvoltage lower than that of a cathode which contains soft iron as a main component. Each of these proposed activated cathodes is formed of a metal substrate of soft iron or the like which is coated with a material formed of metal element of nickel, cobalt, platinum or groups thereof, a mixture thereof, or oxides thereof. An electroplating method, an electroless plating method, a dispersive electroplating method, a thermal spraying method, an immersing method or the like has been proposed as a method of forming the above metal coating on the metal substrate. For example, there have been proposed an activated cathode on which Raney nickel element is plated or a composite of Raney nickel and hydrogen occluding (storage) material is plated (as disclosed in Japanese Post-examined Patent Application Nos. Sho-61-12032 and Sho-61-36590), and an activated cathode on which nickel oxide or cobalt oxide is thermally sprayed (as disclosed in Japanese Post-examined Patent Application Nos. Sho-63-64518 and Hei-3-35387).
The following features are the most important factors for the activated cathodes. It is needless to say that the overvoltage can be kept to a small value, and no deterioration occurs in the activated cathodes when an electrolytic cell is stopped or dismantled. In addition, when the activated cathodes are used with zero gap in an ion-exchange membrane type electrolytic cell, the ion-exchange membrane is not contaminated with a coating material such as nickel or the like, and a manufacturing cost is low.
In the above proposed methods to achieve a low hydrogen overvoltage as described above, for example, the activated cathode on which Raney nickel element is plated or a composite of Raney nickel and hydrogen occluding material is plated can keep a low hydrogen overvoltage, however, this type of activated cathode has disadvantages that the ion-exchange membrane is contaminated with nickel and that when an electrolytic cell is dismantled, the activated cathode is brought into contact with air to be oxidized, so that ignition (spontaneous combustion) occurs in the activated cathode or the activated cathode is deteriorated. On the other hand, in the case of the activated cathode on which nickel oxide or cobalt oxide is thermally sprayed, it is not oxidized and deteriorated even when it is brought into contact with air. However, the latter type activated cathode (thermally sprayed with nickel oxide or cobalt oxide) has a higher hydrogen overvoltage than that of the activated cathode (plated with Raney nickel element or the like), and the hydrogen overvoltage increases with time lapse.
In addition to the above types activated cathodes, an activated cathode having a coating film which is formed of metals of platinum group is also proposed as an activated cathode having a low hydrogen overvoltage. However, this activated cathode has a disadvantage that adhesion of the platinum group metal such as platinum, ruthenium or the like to a metal substrate is weak and thus it is liable to be exfoliated from the metal substrate because the platinum group metal is merely coated on the surface of the smooth metal substrate. In addition, a large amount of platinum group metal is needed to further enhance surface activity, and thus this induces an economical problem. In view of the foregoing problems, no sufficient consideration has been hitherto made on the activation cathode which contains platinum group metal as a main component.